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Standard Square Drives for Socket Tools ASA B5.38-1958

General Engineering Reviews | Engineering Applications and Design

The following is the standard size and dimensions for American square tool drives per ASA B5.38.

Note: All dimensions are in inches

External Drive End
Drive
Nominal Size
Design
Am
Bm
Max
Cm
Dm
Em
M in
Fm
Max
Rm
Max
Max
Min
Max
Min
Max
Min

1/⁄4

A

0.252

0.247

0.330

0.312

0.265

0.165

0.153

0.078

0.015

3/ 8

A

0.377

0.372

0.500

0.438

0.406

0.227

0.215

0.156

0.031

1/ 2

A

0.502

0.497

0.665

0.625

0.531

0.321

0.309

0.187

0.031

5/8

A

0.627

0.622

0.834

0.656

0.594

0.321

0.309

0.187

0.047

3/4

B

0.752

0.747

1.000

0.938

0.750

0.415

0.403

0.216

0.047

1

B

1.002

0.997

1.340

1.125

1.000

0.602

0.590

0.234

0.063

1 1/2

B

1.503

1.498

1.968

1.625

1.562

0.653

0.641

0.310

0.094

Internal Drive End
Drive
Nominal Size
Design
Af
Bf
Min
Df
Ef
Min
Rf
Max
Max
Min
Max
Min

1⁄4

A

0.258

0.253

0.335

0.159

0.147

0.090

3⁄8

A

0.383

0.378

0.505

0.221

0.209

0.170

1⁄2

A

0.508

0.503

0.670

0.315

0.303

0.201

5⁄8

A

0.633

0.628

0.839

0.315

0.303

0.201

3⁄4

B

0.758

0.753

1.005

0.409

0.397

0.216

0.047

1

B

1.009

1.004

1.350

0.596

0.584

0.234

0.062

1 1/⁄2

B

1.510

1.505

1.983

0.647

0.635

0.310

0.125

  • The radius (Rm) located at the shoulder of male tang precludes use of minimum diameter crosshole
    in socket (Ef), unless female drive end is chamfered (shown as optional).
  • If female drive end is not chamfered, socket cross-hole diameter (Ef) is increased to compensate
    for fillet radius Rm, max.
  • Minimum clearance across flats male to female is 0.001 inch through 3⁄4-inch size; 0.002 inch in 1-
    and 11⁄2-inch sizes. For impact wrenches Am should be held as close to maximum as practical.
  • Cf, min. for both designs A and B should be equal to Cm, max.

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